Yes you can join malmshodes, it's first come first serve for monsters and I'm counting fire rodan and rodan as one monster. and Mothra leo and Mothra are the same here as well.
also to Zerofighter two things, one Godzilla is already taken, so is space godzilla, two the X-virus isn't whats connects them to the monsters, it's when the monster's blood is injected inside them that they gain the connection through the X-virus mutating the monster's dna.

 here is the list. Mothra, Rodan, Anguirus, King Caesar, Manda, Zilla, King Kong, Ebirah, Baragon, Varan, Titanosaurus, Battra,Megaguirus, Biolatte, Gorosaurus.

so far the taken monsters are: fire rodan/rodan, Gorosaurus, Godzilla, SpaceGodzilla, Mothra.